This invention relates to a device comprising:
a load which can be powered by a rechargeable battery;
a power-supply circuit for charging the battery and/or powering the load; and
first detection means for detecting a battery voltage substantially equal to a first threshold value.
In the present application "battery" is to be understood to mean a unit comprising one or more cells, such as nickel-cadmium cells.
Such a device is constituted by, for example, a domestic appliance which can be battery-powered or which can be mains-powered by means of a power-supply circuit. The battery can be recharged via the power-supply circuit when the appliance is switched off. In particular, such a device may be constituted by a rechargeable electric shaver.
Such a device is known from European Patent Application No. 110,466, which corresponds to U.S. Pat. No. 4,536,757. In said known device the first detection means are employed for generating a signal which informs the user that the batteries are almost drained and have to be recharged before long. The first threshold value is then selected in such a way that, for example, at least one period of use of approximately five minutes is available after detection of this value. In practice it is found that many users recharge the batteries immediately after the period of use in which the nearly-discharged indication occurs. As a result of this the batteries are never discharged completely. However, in the long run the battery material becomes inert, causing the battery capacity to decrease. As a result of this, the time still available for use after detection of the first threshold value decreases, so that ultimately a full period of use is not available any more. Moreover, the battery capacity also decreases as a result of aging, so that the time still available for use after detection of the first threshold value also decreases. In addition, in the known device the time still available for use depends on the load of the device. A copending U.S. application Ser. No. 121,445 (11/16/87) describes a related battery powered device in which the time still available for use, after a nearly-discharged indication, remains substantially constant.